Penn Central freight trains stopped in the early 70's from Rochester thru Saginaw to Mackinaw City. The coal trains to Bay City beat the railroad to death, but PC couldn't afford to rebuild it, so when Conrail came on board on April 1, 1976, the line went up for sale/abandonment. GTW purchased some segments around Oxford, Lapeer, Saginaw/Bay City/Midland. D&M bought some short sections around Grayling/Gaylord, and the state of Michigan took claim to sections and leased them to other short lines to run, one being Tuscola Saginaw Bay, which operated trains on the Saginaw to Millington portion. The PC trackage at Lapeer that GTW took over was the short section that is now LIRR, plus they also went north about a mile to a lumber yard. GTW ran the spurs until 1999, when Adrian and Blissfield took posession of the line south. The one north to the lumber yard was abandoned and removed in the 80's. There was quite a bit of rail cars moved on the south part until the bridge you mentioned became a problem. There was a chemical plant, and a large warehouse along with the American Standard plastic plant that kept the rails rust free. Now, it is just the AS factory that uses the line. One note of interest, the last actual move on the line north out of Lapeer came in 1976 when a special engine move to Columbiaville using the GTW bicentennial engine #1776 worked out a deal with Penn Central Properties Inc to run that engine there to celebrate the upcoming 100 year anniversary of Columbiavilles existence of being a village. Clink on the link below for that story.
http://www.michiganrailroads.com/stories/5287-helping-columbiaville-celebrate-the-bi-centennial